1. Field of the Invention
This invention relates to input/output apparatus for interfacing field wiring with an electronic control or monitoring system. More particularly, it relates to a modular system with redundancy and a built-in test capability.
2. Background Information
Electronic control systems and monitoring systems such as process controls systems require communication with the outside world. This includes receiving inputs such as commands and process conditions provided by various sensors. transducers and/or contacts, and generating output such as control signals, status and results. This interaction is provided by input/output (I/O) hardware or firmware. The signals pass through the I/O can be analog or digital. Typically, the I/O includes signal processing circuitry which, for example, can limit the range of analog signals, provide filtering, amplification or attenuation, surge protection, isolation, pulse shaping for digital signals, analog to digital or digital-to-analog conversion and other signal conditioning.
Process control and monitoring systems for high integrity applications often require capabilities such as redundancy and on-line testability to ensure high reliability and availability. I/O redundancy requires the capability to diagnose that a problem has occurred with an I/O signal and then to be able to switch the field connections to back-up I/O circuitry of the same type for signal processing. On-line testability functions require the capability to disconnect the field connections from an I/O circuit and substitute simulated test signals. Such redundancy and testing capabilities are commonly required in nuclear and chemical processing industries. These functions typically add significant cost to I/O sub-systems and require specialized hardware and software.
There is a need for improved I/O systems for field wiring, particularly more cost-effective arrangements for implementing redundancy and testability functions.